The invention relates to an endoprothesis of the nucleus pulposus. The endoprothesis is inserted between the base plate and the cover plate of two adjacent vertebrae bodies (corpus vertebrae). The endoprothesis has multiple parts.
There are known a series of devices for replacing degenerated, damaged or destroyed intervertebral discs, or at least are intended for replaicng such discs. There are devices, as described in the U.S. Pat. No. 4,401,112, which insure only pressure absorbtion, without thereby restoring physiological mobility. It is known to replace a pathologically changed intervertebral disc, whereby the cavity resulting from the surgical removal of the nucleus pulposus is filled with silicone rubber which polymerizes out in situ. U.S. Pat. No. 4,349,921 describes a pin-shaped intervertebral disc prothesis. There are also known intervertebral disc protheses of multiple parts made of metal. or combinations of metal and plastic material, or plastic material, each one consisting of an upper part and a lower part with a push-buttonlike, disc or spherical shaped intermediate piece (DE-PS No. 3,023,353; DE-OS No. 2,263,842; CH-PS No. 624,575; CH-PS No. 640,131). U.S. Pat. No. 4,309,777 and SU-PS No. 895,433 depict special constructions.
The known devices have the disadvantage of either not replacing the functions of a nucleus pulposus, or doing so only partially, or only insufficiently. Working cup-shaped support sockets into the spongiosa of the vertebrae bodies has the disadvantage of requiring to some extent protracted manipulations on the patient. Furthermore, it is not desirable that the outer layer of the vertebrae bodies, which is relatively hard in comparison to the spongiosa, is destroyed or perforated. Because of the great contact stresses, the irregular pressure distribution and/or the inflexibility of the materials, there is not only the danger of causing necrosis or bone resorptions, but the vertebrae bodies are also exposed to possible mechanical destruction. Further difficulties result from the surgical techniques which have to be used and from the fact that the safety requirements regarding positional changes are not met with the use of the intervertebral disc endoprothesis.
The object of the invention is to provide an endoprothesis of the nucleus pulposus providing utmost full value, which insures spacing maintenance or restoration and a physiological mobility in the affected section of the spinal column. Furthermore, it also has to be guaranteed operable over a long implant period at utmost safety in regard to positional changes.
The object of the invention is based on the technical problem of providing an endoprothesis of the nucleus pulposus which can be biochemically and biomechanically tolerated, which has stability of shape during pressure absorption, which can be implanted without sophisticated work on the adjacent vertebrae bodies, and which eliminates a mechanical destruction of the adjacent vertebrae bodies.